In fill-level measuring devices or fill level sensors that operate according to the FMCW method, a frequency-modulated signal is radiated, with the use of a transmitting/receiving unit, in the direction of the medium to be measured, is reflected by said medium, and is received back at the fill-level measuring device by way of the transmitting/receiving unit.
Depending on the required conditions of service, various carrier waves can be considered for use in the field of fill-level measuring technology. Apart from acoustic waves, all kinds of electromagnetic waves are of particular importance, wherein in particular the frequency ranges of optical waves and radar waves are of technical relevance.
FIG. 1 shows the technical implementation of a fill-level measuring device according to the FMCW method. The modulation device 1 generates a modulation signal, which in an ideal case rises in a linear manner over time, which modulation signal is transmitted to a voltage-controlled oscillator (VCO) 2. At the output of the VCO 2 in the idealised system a linearly frequency-modulated signal arises, which is transmitted by way of a transmitter/receiver filter circuit 5 to the antenna 6 and is radiated from said antenna in the direction of the medium 7 to be measured. The receive signal that has been delayed in time by the transit time to the feed material and back to the sensor is separated, in the transmitter/receiver filter circuit 5, from the transmit signal, and is fed to the mixer 8. Within the mixer 8 the receive signal is mixed with the transmit signal that is present at the same point in time and subsequently fed to an analog low-pass filtering device 9, which eliminates undesirable signal components. In this manner an intermediate-frequency signal (ZF) arises, whose frequency is proportional to the distance between the sensor and the feed material surface, which distance is to be measured.
For more precise analysis of the intermediate-frequency signal, said signal is digitalised by means of an analog/digital converter (A/D) and is conveyed to a microprocessor system (μP) for further evaluation. Within the microprocessor system (μP) specialised signal processing algorithms process the incoming data stream. Normally, the digitally read intermediate-frequency signal (ZF) is transferred by means of fast Fourier transformation (FFT) to the spectral region where precise determination and measuring of the frequency components caused by the feed material surface can be carried out. The distance value determined by the μP can be provided to a higher-order control device or measured-value display by means of a communication device (KE), not shown, for example by means of a 4 . . . 20 mA line.
In actually constructed FMCW systems the idealised conditions may at best be achieved by approximation. In this context generating a transmit signal whose frequency is to depend on the time in a linear manner may provide problems. According to the arrangement shown in FIG. 1, both the modulation device (MO) and the voltage-controlled oscillator (VCO) have a direct influence on the linearity of the transmit signal. The linearity of the modulation device may be ensured by comparatively simple technical measures. In contrast to this, the characteristic curve of the voltage-controlled oscillator (VCO), whose output frequency generally has no linear interrelationship with the control voltage, may be subject to a host of different influences, wherein in particular series scatter, operating temperature and ageing effect are to be taken into account.
The measuring errors resulting from this non-linearity may have a negative effect on the system sensitivity, measuring accuracy and selectivity between adjacent echoes, wherein the respective inaccuracies increase markedly as the distance from the fill level to be measured increases.
In order to compensate for the above-mentioned non-linearities and the associated measuring accuracies, various methods may be used.
In DE 19713967 a method for correcting the non-linearity of the voltage-controlled oscillator (VCO) is described. According to this method, the output frequency of the VCO is determined as a function of the particular modulation voltage that is present during the actual measuring in a suitable evaluation unit. Based on this characteristic curve, for the subsequent measuring cycle a gradient of the modulation voltage u that differs from the linear function is determined, which gradient is suitable to compensate for the non-linearity of the VCO such that at its output an ideal frequency gradient over time arises.
Furthermore, from EP 1353194 methods may be known which from measuring the width and the amplitude of an echo situated in the measuring region carry out a correction of the modulation voltage gradient. According to the invention, the modulation signal that is used for this is described in a form of a polynomial of order two. Adapting the modulation signal gradient takes place by changing the coefficients of this polynomial.
Furthermore, for correcting the non-linearity, methods are used that provide for a reference branch for the signal, which reference branch is integrated in the sensor. EP 0848829 proposes that by means of a SAW line element a defined echo be generated in the reference branch at a distance that is known in advance. The intermediate frequency generated in the reference branch is subject to the same inadequacies caused by the non-linearity of the frequency sweep as is the case with the measuring signal acquired in the measuring branch. If the measuring signal is scanned according to the method in relation to fixedly defined phase angles of the reference signal, the errors caused by the non-linearity may be eliminated, so that the digital form of the measuring signal becomes identical to that of an idealised system.
A further-reaching embodiment of a correction device combines the above-mentioned methods. In WO 98/38525, by means of a detector device, the period duration is continuously determined from the reference signal generated by a delay device, with the period duration being compared to the desired-period duration that is known in advance. From the resulting deviations, iteratively, a suitable modulation voltage gradient u(t) is determined, which in turn is used to drive the VCO. In the interaction of the correction device, the modulator and the VCO, a signal as would be expected in an idealised system arises in this way at the output of the VCO.
In various respects the correction methods presented so far may be associated with disadvantages.
For example, methods that aim at correcting the non-linearity of the VCO by providing a suitable non-linear modulation signal may either involve increased component expenditure if generating the correction signal is implemented by a separate hardware unit, or alternatively may involve an increased processor load during the actual measuring procedure. In view of the limited energy resources of modern two-conductor sensors the increased output requirements may have a particularly negative effect, especially during the period when the actual measuring takes place, in that a large part of the available energy needs to be used for generating the carrier wave that is necessary in this system.
Moreover, various concepts involve the design of a second reference branch that is used exclusively for internal calibration. Within the reference branch an artificial echo is generated by means of a delay element that has been defined beforehand. From a technical point of view, the delay times required are usually achieved with the use of integrated SAW components. Apart from the additional circuit expenditure in implementing the reference branch, these concepts are associated with disadvantages in particular due to the strong temperature dependency of the SAW component, which temperature dependency in turn needs to be corrected with the use of a temperature sensor.
Furthermore, measuring the width and the amplitude of the intermediate-frequency spectrum, as is proposed in EP 1353194, may be applicable due to the large number of echoes to be expected in the context of fill level measuring.